Liquid catalysts offer many advantages over traditional solid-supported catalysts for the gas phase polymerization of olefins. Feeding of liquid catalysts into reactors has, however, often led to reactor and/or nozzle fouling. Traditional nozzles for spraying liquids,, such as gas assisted nozzles and conventional two-fluid nozzles, require critical coordination of the flow rates of the gas and liquid for satisfactory performance. Flow rates of both the components and the mixture are functions of the nozzle design, particularly the orifice diameter and gas mixing site. Generally, traditional nozzles deliver dense, high velocity sprays immediately downstream of the nozzle exit. The density and velocity of the spray causes it to tend to deposit on the resin in the fluidized bed, leading to accelerated polymerization on the surface of the resin particles already formed. Continued coating with catalyst and the subsequent surface reaction can lead to the formation of resin particles too large to be fluidized in the bed, in turn leading to shut-down of the reactor, a very expensive event. Growth of resin particles from agglomeration effects can also occur due to concentrated catalyst activity. A high initial reaction rate increases the temperature of the young polymer particles, causing them to approach temperatures near, or in excess of, the resin softening temperature. The soft, or molten, resin can adhere to other particles in the bed, resulting in agglomeration and uncontrolled particle growth.
Poor control over catalyst distribution can also lead to unacceptably high concentrations of fine resin particles, which tend to increase the undesirable effects of static electricity, always a potential problem in the reactor. As is known, static charges lead to unwanted accumulations of resin and sheeting. Fine particles also can lead to fouling of the recycle piping, compressor, and heat exchanger.
The use of liquid catalysts in fluidized bed olefin polymerization is discussed in Brady et al U.S. Pat. No. 5,317,036 and in Goode and Williams U.S. Pat. No. 5,693.727, both of which are incorporated herein by reference. See also Keller et al U.S. Pat. No. 5,744,556.
Ultrasonic liquid atomizers are known. See, for example, Berger's U.S. Pat. No. 4,655,393 and Tsai's U.S. Pat. No. 5,687,905, which uses concentric gas introduction to assist in atomization.
Ultrasonic energy has been used to make olefin polymerization catalyst components--see U.S. Pat. No. 4,730,071, col. 1 lines 52-53 and examples 1, 4, and 5; col. 4, lines 19-20; U.S. Pat. No. 5,455,366 col. 20, line 20, U.S. Pat. No. 3,979,370, col. 3 line 13; U.S. Pat. No. 5,559,199, col. 38 line 42; U.S. Pat. No. 5,830,821, col. 18 line 62, and U.S. Pat. No. 5,780,562, col. 16, line 48. However, these processes generally involve the use of ultrasonic baths or dispersions or occasionally breaking up solids. Ultrasonic nozzles are suggested for making polymerization catalysts in U.S. Pat. No. 5,215,949.
Liquid catalysts have been fed to a combustion reaction zone--see U.S. Pat. No. 5,386,690, col. 5 lines 1-8; in four related U.S. Pat. Nos. 5,804,677 col. 13, line 42), 5,733,510 (col. 13, line 44), 5,668,228 (col. 13, line 44) and 5,541,270 (col. 13, line 40) a liquid recycle in olefin polymerization is assisted with ultrasonic nozzles.
Methylaluminoxane was fed together with ethylene through an ultrasonic nozzle into a polymerization reactor, which resulted in "no activity from the zirconium sites"--page 26, WO94/14856.
Many conventional nozzles provide unbroken ligaments of liquid from the nozzle rather than discrete droplets if all conditions are not right--for example, a minimum flow rate. Where high activity solution catalysts are to be fed, it has been observed that substantial amounts of diluent, such as isopentane, must be used to maintain liquid flow rates above the critical value in order to assure droplet formation.